In recent years, electrical storage systems for information-related equipment or telecommunication equipment, i.e., electrical storage systems for equipment having a small size and requiring a high energy density, such as personal computers, video cameras, digital still cameras and cellular phones, as well as electrical storage systems for equipment having a large size and requiring a high electric power, such as electric automobiles, hybrid vehicles, auxiliary power supplies for fuel cell vehicles and electricity storages, have been attracting attentions. As one of the candidates, non-aqueous electrolyte batteries have been actively developed, such as lithium ion batteries, lithium batteries, lithium ion capacitors, etc.
In general, in those non-aqueous electrolyte batteries, a non-aqueous electrolyte containing a non-aqueous solvent and a solute has been used as an ion conductor. The structure of the non-aqueous electrolyte is as follows. An aprotic mixed solvent in which one or several kinds selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate had been mixed has been used as a non-aqueous solvent. A Lithium salt represented by, for example, LiPF6, LiBF4, (CF3SO2)2NLi, (C2F5SO2)2NLi, etc. has been used as a solute.
Until no as a means for improving durability of non-aqueous electrolyte batteries, such as cycle characteristic and high temperature preservability, optimization of various constituent elements of a battery, such as active materials of cathode and anode, etc., has been considered. Technology related to non-aqueous electrolytes is not the exception, either. It has been suggested to suppress deterioration by various additives, deterioration which is caused by decomposing an electrolyte on the surface of active cathode and anode. For example, in a patent document 1, it has been described that high temperature cycle characteristic is improved by an effect of a film formed on an electrode interface when lithium difluorophosphate (LiPO2F2) is added to an electrolyte.
As a method for manufacturing lithium difluorophosphate used as an additive, a method in which halides other than fluoride, LiPF6 and water are reacted in a non-aqueous solvent has been known (see a patent document 2).
In addition, in a patent document 3, for example, after reacting diphosphorus pentoxide, lithium hexafluorophosphate and hydrogen fluoride, a solution is concentrated, following which it is cooled down, and the crystal of lithium difluorophosphate is obtained. In a patent document 4, after reacting difluorophosphate and lithium chloride, a solution is cooled down, and the crystal of lithium difluorophosphate is precipitated.
In the method described in the patent document 2, a reaction liquid containing unreacted LiPF6 and the produced lithium difluorophosphate can be used for preparing an electrolyte. However, there is a possibility that as a by-product produced by decomposing LiHPO3F, H2PO4 and LiPF6 is mixed, and it has therefore been required to obtain lithium difluorophosphate having higher purity. As its method, it has been considered to purify lithium difluorophosphate by obtaining the powder of lithium difluorophosphate by precipitating the crystal of lithium difluorophosphate from a solution.
However, lithium difluorophosphate tends to be gel due to hydrogen bound, and a method for manufacturing lithium difluorophosphate powder from a solution, method which is capable of being used for industrial production, has therefore not been established. For example, in the patent documents 3 and 4, by a concentration method in which a solvent is removed and a cooling method in which a temperature of a solvent decreases, it is possible to precipitate several tens of grams of lithium difluorophosphate in the level of a laboratory by solid-liquid separation, ignoring a device load. However, the precipitated lithium difluorophosphate is turned into gel with these methods, and consequently, effective recovery of lithium difluorophosphate often becomes difficult, because the solid-liquid separation of several kilograms or greater of lithium difluorophosphate takes a long time.